Present understanding of antibodies and their related genes is based almost entirely on information derived from a few mammalian species. The overall goal of this research program is to increase our understanding of the range of diversity in the immune system by examining immunoglobulins and immunoglobulin genes in the developing amphibian, Xenopus laevis. We believe that such investigations will not only broaden our knowledge of immune mechanisms in diverse organisms, but may also clarify aspects of the immune response in humans. We will isolate and characterize immunoglobulin-related clones in a cDNA expression library that was prepared from mitogen- stimulated Xenopus splenocytes. The library will be screened with antibodies prepared against the two major immunoglobulin classes in this species, IgM and IgY, and also with nucleic acid probes derived from the cDNA clones. The nucleotide sequences of cDNA inserts encoding diverse heavy and light chains will be determined and correlated with partial amino acid sequence information derived from the isolated polypeptide chains. The results are expected to clarify the number of heavy and light chain isotypes. The data may also provide information about the extent of sequence diversity in the variable regions of these immunoglobulins. Nucleic acid probes derived from cDNA clones isolated and characterized in the first phase of this study will be used to analyze the arrangement and rearrangement of immunoglobulin genes in Xenopus laevis. This information may provide insight into similarities and differences between the immune response in amphibians and that in mammals. The expression of immunoglobulin genes at various stages of Xenopus development will be examined by in situ hybridization with nucleic acid probes corresponding to different heavy and light chain variable and constant regions. The amino acid sequences of immunoglobulin heavy and light chains in Xenopus laevis will be compared to those of other species. Phylogenetic trees will be constructed in order to gain insight into the evolution of this family of proteins.